1. Field of the Invention
The present invention pertains to methods and apparatus for providing fluid flow control assemblies for fluid-handling machinery. More particularly, the present invention relates to techniques for controlling the axial clamping forces on adjustable radial blade assemblies in fluid-operable systems. The present invention finds particular application to radial turbines and compressors wherein variable pressure profiles across the blades and annular parallel rings cause wide variations in the ring clamping forces on the blades as the orientation of the blades is altered to change the width of the interblade flow passages.
2. Description of the Prior Art
In the case of radial turbines and some other fluid-handling rotating machinery, pressurized fluid is communicated into the turbine wheel, or rotor, through an array of circumferentially arranged nozzles. The flow of fluid through the nozzle assembly may be varied by pivotally adjusting the nozzle blades so as to vary the flow area passageway between adjacent nozzle blades. Similarly adjustable diffuser blades or vanes may be arranged in a circumferential array in a compressor.
In one type of a variable nozzle turbine, the nozzle passages are formed by a collection of rotatable blades positioned between a pair of axially-spaced parallel rings. Complimentary portions of adjacent nozzle blades, along with portions of the adjacent ring surfaces, form the nozzle passages. Each blade is pivotally mounted on a pin fixed to one of the rings, and a second pin affixed to the opposite ring engages an offset cam slot in the nozzle blade. Rotation of the second, or actuator, ring effects a camming operation to rotate the blades in unison around their respective pivot pins to alter the distance between adjacent blades and, therefore, to vary the flow area passageway between adjacent nozzle blades.
U.S. Pat. No. 3,232,581 discloses a variable nozzle arrangement in which the pressure of the inlet fluid is utilized to generate appropriate clamping forces among the nozzle assembly components, such forces being sufficient to prevent leakage between the nozzle blade end walls and annular ring surfaces, but not so great as to prevent or impede the operation of the nozzle adjustment mechanism. The clamping force is determined at least in part by the selection of an effective seal diameter located generally between the minimum and maximum diameters on the outside of the annular nozzle actuator ring. Such seal operates to separate high pressure inlet fluid from the lower pressure fluid at the exit of the nozzles and within the turbine rotor housing. The high and low pressure zones thus separated act on their respective outside areas of the annular actuator ring. The resultant force acting on the outside of the ring is opposed by the resultant force determined by the pressure profile existing within the nozzle assembly and acting on the inside exposed area of the actuator ring. The effective seal diameter is thus chosen such that a net compression, or clamping force, of sufficient magnitude will be created for the purpose of sealing the nozzle blade end walls against the inside annular surfaces.
U.S. Pat. No. 3,495,921 discloses a variable nozzle arrangement in which the inside surfaces of the annular rings have been relieved slightly. This feature is helpful in overcoming certain limitations associated with control of the clamping force on the nozzle assembly merely by selection of an effective outside seal diameter as described above in U.S. Pat. No. 3,232,581. Because of variation in the opposing resultant force pattern acting upon the inside annular walls of the nozzle assembly as the nozzle blade orientation is adjusted to contol the flow, the net compressional clamping force does not remain constant. In the selection of an effective outside sealing diameter, consideration is given to maintaining at least a minimum clamping force with the nozzle blades in a closed position. As the nozzles are opened, changes in the resultant force pattern within the assembly will act in a manner to decrease opposition to the compression force acting on the outside of the actuator ring, thereby resulting in an increase in the net clamping force. In applications utilizing high inlet pressures, the clamping force may thus increase to such magnitudes as would impede operation of the nozzle adjustment mechanism.
The improvement introduced in U.S. Pat. No. 3,495,921 involves controlling the variation in the resultant force pattern acting on the inside of the annular rings by tapering or otherwise relieving the annular rings such that the exposed inner surfaces of the annular rings are subject to essentially constant and equivalent pressures regardless of blade orientation.